As the spectacle of the Salem witch trials played out in Massachusetts in 1692, concluding with the execution of 20 innocents, New Yorkers, indulging a finer sense of civilization, condemned the hysterical confessions and lethal injustice of their New England cousins. All the more stunning, then, that half a century later, New York underwent its own bout of hysteria, outstripping Salem in both body count and self-righteous malice. In 1741, a "witch hunt" in Manhattan put 30 slaves and four whites to death by gibbet and burning at the stake.

The government-run frenzy was driven by fears and rumors not of witches but of a slave revolt. It was conducted at City Hall in a star chamber proceeding fed by a fanatical prosecutor and his informer-driven roundups. The slaves were reduced to pleading for life, not liberty. The fact that such an atrocity has been forgotten makes a modern New Yorker pause in dark wonder at the bloody histories buried under the city streets. In this instance, a parody of justice was interred with the bones of the slaves just a short stroll north of City Hall. The victims were cast into the oblivion of the Negroes Burial Ground, a 17th-century cemetery accidentally rediscovered 14 years ago. It was the resting place of an estimated 20,000 black New Yorkers, slave and free.

The task taken up by the Harvard historian Jill Lepore in "New York Burning" is to disinter this grisly shard of colonial history and try to separate fact from fear, truth from rumor. The only frustrations in reading this gripping book are the paucity of primary materials and the surfeit of unanswerable questions.

The record shows no firm proof of a slave conspiracy that was ready to explode in a burst of murder, rape and pillage. But it does show that whites' fears of such an apocalypse were palpable. "The Negroes are rising!" was the cry heard after a series of fires during five weeks in March and April of 1741, including one at Fort George, the seat of royal government and the symbol of law and order. A white indentured servant, 16-year-old Mary Burton, was soon obliging investigators with nonstop tales of conspiring slaves. She was lauded by the prosecution as "the happy Instrument of all this Discovery."

Lepore, the author of "The Name of War: King Philip's War and the Origins of American Identity," presents a fascinating social and political history by focusing on the one detailed source that does survive -- the self-aggrandizing, self-vindicating journal of Daniel Horsmanden, a scheming city recorder and judge. It's a document that requires wary parsing, because Horsmanden was the overweening instigator and prosecutor of the entire affair, driven by his concern about a "villainous Confederacy of latent Enemies amongst us." It was he who saw to it that more than 150 men and women were taken from their masters, along with a few accused white ringleaders, and thrown in the City Hall dungeon, just up from the busy Wall Street slave market. Informing and confessing were his only prosecutorial tools, but since those who cooperated might be spared death, he was able to obtain 80 formulaic confessions, which in turn implicated others.

Lepore tersely summarizes Horsmanden's power as an unrepentant sacrificer of defenseless slaves: "He traced their motives, limned their characters and followed their fates." Yet some of the doomed managed to show individuality, even in Horsmanden's skewed account. "Dr. Harry," arrested because slaves were forbidden to practice medicine, was the perfect victim for the proofless prosecution, and was portrayed as diabolically intent on poisoning whites. The gritty Dr. Harry rebuffed his last chance to survive, declaring at the stake, "It signified nothing to confess."

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The subtext of this book is that early New York, with 20 percent of its residents enslaved, was a tinderbox in which political repression was employed to deal with the fear and shame of human servitude. One revolt in 1712 took the lives of nine whites and about 25 slaves, and in the colonies overall there were at least as many imagined uprisings as real ones. Lepore juxtaposes colonial America's stirring polemics about freedom with Samuel Johnson's demurrer from London: "How is it that we hear the loudest yelps for liberty among the drivers of negroes?"

"New York Burning" details the deep insecurity of a white majority, a killing force eventually eased when the propertied class tallied the valuable holdings being burned and hanged daily before angry crowds. "To execute slaves was to burn money," Lepore notes, as she describes how the City Council sought to offset the budget strain of Horsmanden's bloody dragnet by demanding crown subsidies.

In modern language, Horsmanden was a "kiss-up, kick-down" politician, dealing death with grand ambition. But as happens today with stories in our 24/7 news cycle, popular interest in his crusade faded once slave corpses were rotting in chains and public worry shifted to an equally questionable Papist plot. "We seem to be easier as to the Thoughts of the Negroes," John Peter Zenger's Weekly Journal reported, as if a mere heat wave had passed. New Yorkers began questioning whether all those confessions were believable.

An anonymous letter from Boston enraged city fathers by recalling New York's mockery of New England for the Salem trials. Lepore relishes this precious addendum to Horsmanden's record -- a contemporary witness skewering New York's slave terror as "the merciless flames of an Imaginary Plot." The letter warned the city against "making Bonfires of the Negros & perhaps thereby loading yourselves with greater Guilt than theirs." As a tool of history, the letter trumps Horsmanden's journal. But it burdens any modern attempt at strolling the city's storied streets in a spirit of innocence.